Xylenes are found as a mixture of the o-xylene, m-xylene and p-xylene and are found as components in petroleum reformates, or coal pyrolysis tar distillates and the like. Xylenes are generally recovered and separated by solvent extraction, distillation and/or fractional crystallization. For example, p-xylene may be separated from other C.sub.8 aromatics by fractional crystallization.
Although o-xylene, m-xylene and p-xylene are as useful as chemical reagents, solvents, etc., p-xylene is particularly valuable, since p-xylene finds wide use in the manufacture of polyester. As a result of this significant commercial interest in p-xylene several processes have been developed to provide p-xylene. One such process is Octafining. In Octafining a C.sub.8 aromatics mixture is subjected to a series of processing separation steps involving isomerization of a C.sub.8 fraction depleted in the desired C.sub.8 product component which is then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics recycled to a product separation step. The isomerizer unit in Octafining itself is most simply described as a single reactor catalytic converter. The catalyst usually contains a small amount of platinum and an acid component which can be mordenite, silica-alumina and the like, and the reaction is carried out in a hydrogen atmosphere.
Xylene isomerization includes processes for the isomerization of monocyclic methyl-substituted aromatic hydrocarbon feedstocks. Several processes are employed commercially for the isomerization and include, but are not limited to, "Octafining" and the "LTI" process. These processes are only two of the processes which may be employed for xylene isomerization.
The design of an Octafiner process unit is generally characterized in the literature by the following specification ranges:
______________________________________ Process Conditions ______________________________________ Reactor Pressure 175 to 225 PSIG Reactor Inlet 750-900.degree. F. Temperature Range Heat of Reaction Nil Liquid Hourly 0.6 to 1.6 Vol/Vol/Hr. Space Velocity Number of Reactors 1 Downflow Catalyst Pt/silica-alumina Catalyst Bed Depth 11 to 15 Feet Catalyst Density 38 Lb/Cu. Ft Recycle Circulation 7.0 to 14.0 Moles Hydrogen/Mol Hydrocarbon Feed Maximum Catalyst 20 Pressure Drop. PSI ______________________________________
Octafining is extensively discussed in the literature as exemplified by the following:
1. Pitts, P. M. Connor, J. E., Leun, L. N., Ind. Eng. Chem, 47, 770 (1955). PA0 2. Fowle, M. J. Bent, R. D., Milner, B. E., presented at the Fourth World Petroleum Congress, Rome, Italy, June 1955. PA0 3. Ciapetta, F. G., U.S. Pat. No. 2,550,531. PA0 4. Ciapetta, F. G., And Buck, W. H., U.S. Pat. No. 2,589,189. PA0 5. Octafining Process, Process Issue, Petroleum Refinery, 1st Vol. 38 (1959), No. 11, Nov., p. 278.
The C.sub.8 aromatics feed may be derived from petroleum derivatives, e.g., aromatic gasoline fractions from various processes, including solvent extraction. C.sub.8 aromatics generally comprise ethylbenzene, o-xylene, m-xylene, and p-xylene in amounts of from 10 to 32 wt. % ethylbenzene, 50 wt. % m-xylene, up to 25 wt. % o-xylene and up to 25 wt. % p-xylene. Under the conditions commonly employed for Octafining the calculated thermodynamic equilibria for the various C.sub.8 aromatic isomers are:
______________________________________ Temperature 850.degree. F. ______________________________________ Wt. % ethylbenzene 8.5 Wt. % para-xylene 22.0 Wt. % meta-xylene 48.0 Wt % ortho-xylene 21.5 TOTAL 100.0 ______________________________________
It has been reported that an increase in temperature of 50.degree. F. will increase the equilibrium concentration of ethylbenzene by about 1 wt %, ortho-xylene is not changed and para- and meta-xylenes are both decreased by about 0.5 wt %. (See: U.S. Pat. No. 4,158,676).
In Octafining the process is capable of converting a portion of the ethylbenzene to xylenes and, accordingly, some ethylbenzene can be present in the process. The various processing steps employed in Octafining are well known in the art and will not be further discussed here.
Low Temperature Isomerization (LTI) is described in U.S. Pat. No. 3,377,400, dated Apr. 9, 1968. The LTI process is characterized by the capability of isomerizing xylenes in the liquid phase at relatively low temperatures and without the necessity of having a hydrogen pressure in the isomerization reactor. The zeolite catalyst ages very slowly even without hydrogen or a hydrogenation/dehydrogenation metal component on the catalyst. However, LTI has one disadvantage in that it does not result in the conversion of ethylbenzene. Since the ethylbenzene content of C.sub.8 aromatic fractions is relatively unchanged in LTI operations, such operations generally incur increased costs in capital investment and operating expenses since ethylbenzene must be removed from the process. Because of the minor differences in the boiling points of ethylbenzene and certain of the xylenes, complete removal of ethylbenzene from the charge is extremely expensive. The practical way to remove ethylbenzene is to provide an additional distillation column for removing ethylbenzene. Unfortunately, the incorporation of this additional distillation column involves a significant additional cost and increases the operating expenses for the process.
The instant process relates to novel isomerization catalysts and to their use in isomerization processes to prepare xylene mixtures which are isomerized to an substantially equilibrium distributions or higher in the para-product. In some embodiments meta-xylene is isomerized to para- and ortho-xylenes and/or ethylbenzene is converted to products, e.g., xylenes, to provide process products having a lower ethylbenzene content than the feedstock.